Cyclopropanes protonated

In the C NMR spectrum two signals with unusually small shift values [(07/2)2 5c = 7.7 CH 5c = 10.6] and remarkably large CH coupling constants Jch= 161.9 and 160.1 Hz) indicate a mono-substituted cyclopropane ring A. The protons which belong to this structural unit at = 0.41 (AA ), 0.82 BB ) and 1.60 (M) with typical values for cis couplings 8.1 Hz) and trans couplings 4.9 Hz) of the cyclopropane protons can be identified from the CH COSY plot. 

The additional coupling (9.8 Hz) of the cyclopropane proton A at <5// = 1.60 is the result of a vicinal H atom in the side-chain. This contains a methyl group B, a vinyl group C and an additional substituted ethenyl group D, as may be seen from the one dimensional H and C NMR spectra and from the CH COSY diagram. 

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

While the nuclei of the aromatic segments show the identical signal directions, the cyclopropane protons show characteristic differences. This suggests significantly different spin-density distributions for the cyclopropane moieties of the two species and, thus, different structures. Like the norcaradiene HOMO, the styrene HOMO is antisymmetric at the positions of attachment, suggesting preferred interaction with the antisymmetric cyclopropane HOMO. In the norcaradiene system, the natural structure ( Aj) of the cyclopropane radical cation is altered by the interaction with the diolefin entity. 

Phenylcyclopropane radical cation (9 ) has divergent hyperfine coupling constants for the secondary cyclopropane protons (aptrans = 0.78 mT upcis = 0.07 mT CIDNP, B3LYP/6-31G calculations), apparently because the cis protons are located in a nodal plane. " Similarly, vinylcyclopropane radical cation is... 

Some of the more remarkable effects of strain are found in NMR chemical shifts. Cyclopropane derivatives usually have upheld proton chemical shifts with regard to the corresponding cyclohexane derivatives, whereas cyclobutanes commonly have downfield shifts.The upfield shift for cyclopropane protons have sometimes been attributed to a ring current in the three-membered ring, but there is little evidence for such a phenomenon. The unusual shift for these protons has proven valuable in demonstrating the presence of a three membered ring. 

Fig. 19. A comparison of the CIDNP effects observed for the cyclopropane protons of ch-diphenylcyclopropane (left) and[l 2,9 10]bismethano[2.2]paracyclophane (right) [229]...

A great deal of sense can be made of the experimental data if we assume that there is a competition in any cyclopropane protonation reaction between comer protonation and direct one-step opening with the latter pathway the more favored the greater the driving force for ring-opening. We therefore expect the one-step mechanism to predominate for 4, and for other systems in which a good carbonium ion is formed. When the carbonium ion which would be formed is not especially stable... 

The facility of cyclopropanes to react with proton acids and form protonated cyclopropanes—the stability of the complex formed relative to the stability of the cyclopropane—is the basicity of cyclopropane. Protonated cyclopropanes are well established as discrete structural entities and in special circumstances can be generated under conditions where they have a sufficient lifetime to allow spectral investigation. The complex of proton plus cyclopropane can revert to starting species, i.e. cyclopropane and proton, or undergo further reaction, whether rearrangement or addition of a nucleophile. 

The very small shifts of cyclopropane protons may have troubled you, but they often have 6h 1.0 p.p.m. Compounds 13A and C are simple enough but 13B may have amazed you. It is unsta r s but can be isolated and the two spiro rings sit at right angles to each other. There is again a CH group whose protons are different. 

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